1. Field of the Invention
The invention relates to magnetic resonance (MR) devices such as magnetic resonance imagers and spectrometers, and more particularly, to a method and apparatus for substantially reducing an eddy current disturbance in an MR device having a pulsed magnetic field gradient coil.
2. Description of the Prior Art
MR systems are based on the phenomenon of nuclear magnetic resonance (NMR). When an object is placed in a magnetic field, the field causes the spin vectors of certain types of nuclei having a net magnetic moment (e.g. .sup.1 H, .sup.13 C, .sup.31 P and .sup.23 Na) to orient themselves with respect to the applied field. These nuclear spin vectors, when supplied with the right amount of energy, will reorient themselves in the field in accordance with the applied energy and emit or absorb energy in the process. The energy needed to perturb the nuclear spin vectors is in the radio frequency (RF) range, and the specific frequency depends upon the strength of the magnetic field which is experienced by the nuclei. In MR devices which do not provide electrical control of the spatial positioning of the applied magnetic fields, the sample is placed in a large, uniform, static magnetic field. The sample of MR active material is perturbed by a pulse of radio frequency energy applied to an RF coil, and the frequency of the nuclear spin vectors are sensed by the RF coil and response signals of the perturbation recorded. A measure of signal intensity as a function of resonance frequency or magnetic field at the nucleus (e.g., the free induction decay (FID) signal is obtained and analyzed, in a manner well known, to derive an image or spectroscopic information about the sample.
Imaging and spatially dependent spectroscopic analysis methods carry the above-described MR technique one step further by using magnetic field gradients in addition to a static primary background (main) uniform magnetic field. Since the resonant frequency of the MR active nuclei depends upon the precise magnetic field strength imposed upon it, applied field gradients are used to provide a technique for encoding spatial information into the sensed frequency response signals. MR devices correlate signal intensity at a given frequency with sample concentration and relaxation parameters at a given location. This provides spatial information which is used to make a map or image of the object, based upon signal intensity variations due to concentration and/or relaxation time differences. In a spectrometer, these field gradients allow a spatial selection of a particular portion of the sample object to be analyzed. The field gradients are produced with a set of gradient coils. These coils are often referred to as "pulsed gradient coils" because they are energized by pulses which grade the main field in two or more orthogonal directions.
Imaging the entire body of a patient, for example, typically requires a steady, high homogeneity, main field and highly linear gradients in the range of, for example, 0.1-1.0 gauss/cm with rise and fall times as short as possible, typically on the order of 0.1 to 1.0 milliseconds. An axial gradient (i.e. in the "Z" direction) is typically produced by solenoid coils while radial gradients (which define "X" and "Y" coordinates) are formed by saddle-shaped coils, as well known.
Regardless of the way in which the static background field is produced, for example by a superconducting magnet system, the changing magnetic fields which result from the pulsing of the gradient coils will induce eddy currents in any nearby conducting media. In the case of MR devices using superconducting magnets, most of the eddy currents are induced in the radiations shields which are designed to reduce evaporation of the cryogens. Eddy currents are also induced into the shim coil system and/or RF shield used to decouple the gradient coils from the RF coil.
Eddy currents have an adverse effect on both the spatial and temporal quality of the desired gradient fields. The eddy currents themselves generate a magnetic field which superimposes on the field produced by the gradient coils, thereby disturbing the gradient coil field from its desired level and quality, in both space and time. One observes, for example, an approximately exponential rise and decay of the gradient fields during and after, respectively, the application of a rectangular current pulse to the gradient coil. The result of this perturbation is that the amplitude and phase characteristics of the MR signals sensed by the RF coil are distorted, thereby reducing the accuracy of the spectroscopic analysis or the quality of the generated images. Therefore, it is necessary that the eddy currents be carefully controlled, compensated for or reduced to an insignificant level.
As known to those familiar with MR devices, basically two different approaches are used to eliminate the effects caused by eddy currents. One approach is to provide a self-shielded gradient coil which, in concept, prevents the gradient field from penetrating to the surrounding structure of the main magnet. Thus, the eddy current generated magnetic fields are avoided altogether. U.S. Pat. No. 4,733,189 issued Mar. 22, 1988 is illustrative of this technique and discloses an active shield. This approach suffers from several drawbacks. Firstly, the diameter of the gradient coil is reduced due to the presence of the active shield. This limits the size of the objects which can be investigated. Secondly, the power consumption of the gradient coil is increased due to the close proximity of the active shield to the gradient coils. Furthermore, any eccentricity between the active shield and the gradient coils produces a base field shift during application of the gradient pulse. Since both magnet and gradient amplifier designs have been pushed to the edge of their relevant technology, these drawbacks are quite severe and have limited the wide spread implementation of actively shielded gradient coils.
A second approach is to provide gradient pulse modification in a manner so as to reduce or eliminate the effects of eddy currents. This modification is typically achieved by superimposing a desired gradient pulse shape with a set of mono-exponential functions. Although hardware used to achieve the super is well known (see, for example, U.S. Pat. No. 4,703,275 of G. Neil Holland, incorporated herein by reference), the calculation of the circuit component values has proven to be extremely difficult. U.S. Pat. No. 4,698,591 of Glover et al. (incorporated herein by reference), relates to a method for calculating the exponential functions for deconvolving the desired gradient pulse shape by analyzing the phase characteristics of the free induction decay (FID) signal. After an initial estimate is made of the exponential function chisquared minimization techniques of a Taylor's series expansion about the initial estimate point, is used for defining the multiexponential functions. Since acquisition and analysis of the phase of the FID signal is not a technique conventionally found in an MR device, such acquisition and analysis of the phase of the FID signal must be specifically acquired and processed in the MR device. Additionally, chi-squared minimization techniques and Taylor's series expansion are further signal processing techniques not found in MR devices and therefore must be specially supplied.
Thus, the present invention is directed to a method and apparatus for measuring the effects of eddy currents and accurately calculating coefficients which can be used to cancel or compensate the effects of eddy currents. These coefficients can be used to set the time constant and amplitude characteristics of a pre-emphasis filter of the type used in U.S. Pat. No. 4,703,275 (noted above) or can be used by the software of the MR computer system for initially generating the gradient current pulse with the proper "pre-distorted" leading and trailing edges. The transient nature of the composite field produced by the gradient coil and the eddy currents is evaluated with the inherent high sensitivity of the MR device by studying the FID signals in the frequency domain and offers significant improvements over the Glover technique in that special programming packages are not required for determining the phase of the FID signals nor for analyzing the phase response in order to calculate the amplitude and time constant correction coefficients. In the present invention, standard signal processing techniques already incorporated in the MR device, namely one-dimensional Fourier transformation processing, is utilized for quantifying the effects of the eddy currents.